A silicon photomultiplier (“SiPM”) is a photo-sensing element in which avalanche photodiodes (“APDs”) are two-dimensionally arranged and operated with a reverse-bias voltage that is higher than the breakdown voltage of the APDs in a regime called “Geiger mode.” Since the gain of an APD operating in the Geiger mode is very high, 105 to 106, subtle light of a single photon may be measured.
A resistor with a high resistance called “quenching resistor” is connected in series to each APD. When a single photon enters the APD to cause a Geiger discharge, the current amplification is terminated by a voltage drop caused by the quenching resistor. Therefore, a pulse output signal is obtained. Since each APD in a SiPM operates in the manner described above, if the Geiger discharge is caused in two or more APDs, an output signal with a charge value or pulse wave height value that is the charge value or pulse wave height value of a single Geiger-discharge APD times the number of Geiger-discharge APDs is obtained. Therefore, the number of Geiger-discharge APDs, i.e., the number of photons entering the SiPM, can be measured from the output signal. This enables single photon measurement.
As described above, a SiPM operates with a reverse-bias voltage that is higher than the breakdown voltage of APDs. Therefore, the thickness of a depletion layer of each APD is generally 2 μm to 3 μm, and the reverse-bias voltage is generally 100 V or less. Accordingly, the spectral sensitivity characteristic of the SiPM is highly dependent on the absorption characteristic of silicon, and the SiPM has a sensitivity peak in a range from 400 nm to 600 nm, but is substantially not sensitive to near infrared light in a wavelength band of 800 nm or more.
Compound semiconductor devices, for example, are known as photo-sensing elements having a high sensitivity in near infrared light wavelength band. However, the photo-sensing elements of this kind are still expensive, and complicated to manufacture. Silicon-based photo-sensing elements in which the depletion layer has a thickness as thick as several micrometers to become sensitive to light in near-infrared wavelength band are also known. However, the drive voltage for driving the photo-sensing elements of this kind is as high as several volts, and the APD array in these photo-sensing elements is not minute like that of SiPMs.
Photo-sensing elements of another kind are also known, in which a silicon substrate has a scattering surface on the back side with irregularities made by laser processing to reflect unabsorbed light. However, it is difficult to control the preciseness of the irregularities of the scattering surface to appropriately reflect light in the near-infrared wavelength band. Furthermore, a dedicated laser processing machine and a special process are needed, which may lead to an increase in costs. Mechanically processing a silicon layer included in a diode leads to forming a defective layer, and may cause problems in the electric characteristics of the photodetectors such as stability, yield, and reproducibility. Moreover, in consideration of the extinction coefficient of silicon in the near-infrared wavelength region, the photo-sensing element itself needs to be thinned to about 10 μm. This also increases the number of additional steps and the manufacturing costs.